Process for treating a magnetic iron ore



June 26, 1951 D. N. VEDENSKY HAL 2,558,635

PROCESS FOR TREATING A MAGNETIC IRON ORE I Filed Dec. 15, 194'? 2 Sheets-Sheet 1 CRUSHED ORE ROD MILL MAGNET/C SEPAPATOR TAIL was CONCENTRATE N05 DEMA GNET/ZEIZ BALL MILL CLASS/PIER OVEEFLOW MA GNE TIC SEPARATOR 5 MA 6N5 T/Z E R HYakA UL /c c1. ASS/PIER llll"l OVERFLOW 5P/60T 4 .SP/GOT 3 5P/60T 2 SP/GOT I MAGNET/ZINE MAGNET/ZINE MA 6 NE T/Z l/V' MA GIVE TlZ/NG COIL COIL COIL co/z.

TTTTT HYURAUL C HYDRA UL C HYDRA UL I C H YDRA UL [C C CLASS/PIER CLASS/F/ER CLASS/F/ER CLASS/FIE? TOTAL TOTAL UNDERFLOW OVERFLOW JNVENTORS war/mu /v. VEDENSKY Lzsue u. SEC/MUD, J1:

June 26, 1951 D, N, VEDENSKY HAL 2,558,635

PROCESS FOR TREATING A MAGNETIC IRON ORE Filed Dec. 15, 1947 2 Sheets-Sheet 2 v ROD MILL MAGNET/C SEPARATOR CDIVCENTR'ATE TAIL/N65 DEMAGNET/ZER MAG/VET 1C SEPARATOR CLASS/HER I a E -FL v k BALL MILL TAlL/N68 7 HYDRAULIC. CLASS/PIER OVERFLOW ESP/6 074 5P/60T 3 SP/GOT 2 SP/GOT I MAGNET/Z/NG MAGNET/Z/NG MAGNET/ZINE MAGWET/Z/NE COIL COIL COIL co/L HYDRAULIC HYDRAULIC HYDRAULIC HYDRAULIC H C CLASS/HER CLASS/FIE? cuss/F152 cuss/HER I 1 l A v v v 707.41. TOTAL UNDERFLOW OVEEFLOW HINVENTORS DM/TR/ w. VEDE/VSKY Y1. ESL IE d. BEG/MUD, Jr:

ATTORNEY Patented June 26,1951

UNITED STATES PAT|.=.1 11" O'F FI CE' PROCESS FOR "TREATING A MAGNETIC I IRON ORE Dmitri N. Vedensky, Oakland, calm, and Leslie J. Bcchaud, Jr., Hibbing, Minn., assignors to Hanna Coaland Ore Corporation, a corporation of Delaware Application December 15, 1947, Serial No. 791,866

In the beneficiation of ores in general, the ore must be crushed or ground to an average size at which the valuable'mineral sought breaks free from the surrounding waste material. It is ob-1 vious that such crushing or grinding is necessary inorder to liberate the valuable mineral in almost all cases of such crushing or grinding.

liberation of the valuable mineral is never complete and a certainamount is left locked with worthless waste material. These mixed par-- .ticles, commonly called middling grains, may range in composition from nearly all waste material to nearly all valuable mineral. In any scheme for recovering the liberated valuable mineral, a certain, amount of middling grains will also be recovered, the amount and composition depending, among other things, on the type of recovery treatment used.

vious that the purity of the concentrate pro-.

duced when treating iron ores by magnetic methods is aflected by the quantity'and composition of the'middling particles recovered concurrently with fully liberated valuable mineral particles.

To improve the grade of the magnetic concentrate, it is commonpractice to continue grinding the concentrate to the point whereat the middling particles .are reduced in size and the contained valuable mineral is liberated in a virtually free state so that, in a subsequent magnetic concentration, the gangue portion of the former middling particlesis rejected from the -concentrate. This practice is open to objection on the grounds that to accomplish grinding of the middling particles, the associated free mineral particles must simultaneously be subjected to 5 Claims. (cl. 209 s9) .The present invention enables the undesirable middling particles to be separated from the magnetic concentrate without. the expenditure of energy required for additional ,g'rinding. We

; have found that we can make use of .the differential in settling rates between high .grade iron oxide particles on the one hand,.- and low grade middling particles on the other to efiect an adequate separation. In order to explain fully how this differential in settling rates can be employed to advantage, it is desirablev tov describe typical illustrative fiowschemes embodying the invention for the beneficiation of iron ores involving the use of magnetic separation.

For treating a disseminated ore containing iron oxides, which for the purposes of thisdiscussion v we will assume to be magnetite, one fiowscheme grinding. In other words, in order to accomplish useful work on what may be a small portion of the magnetic concentrate'as provided by the middlings, energy must also be expended on a large portion of otherwise finished material. Yet, in many cases, if this additional grinding energy were not expended and the middlings reduced in size, the magnetic concentrate would be of little value because of the excessive silica content occasioned by the presence of low grade middling particles; generally the silica content of the concentrate must not be over 10% silica as SiOz.

can typically include grinding, the ore tofinely divided form in a ball mill-classifier circuit, the overflow from the classifier going to magnetic separators which produce a tailing to be discarded and a concentrate which, as handledprior to this invention, has been subject to further grinding for liberation ofthe iron values in theconcentrate contained middling particles. This additional grinding step has been eifected in' a ball mill-classifiercircuit with the classifier; overflow going toan additional stage of magnetic separa tion to produce the final finished concentrate;-

.The first magnetic concentrate,of courseycon- .tains: particles ranging in size between that at which the first classifier overflows and finest slime. It is characteristic that the-coarse size ranges of the concentrate containrelativelylow values in iron while the fine sizes are high in iron. The low-iron values in the coarse size ranges are due to the presence of low grade middling particles. Atypical screen analysis of a .ffirst magnetic concentrate made from a classifier. overflow having a maximum particle "sizesuch that all particles pass a mesh screen is shown inTableL- 1 1 TABLE I i Screen analysis of 65 mesh magnetic concentrate Distribu- Wt.. Per Assay Fe Product tlon Fe.

. Cent Per Cent B C Compo. Conc. x00. 00 56. a 100.0 +65 Mesh 0.68 32.5 0. 4 -s5+100 Mesh. 4. 0o 25. 2 1. s -1c0+1s0 Mesh- 9. 06 a2. 3 s. 2 -1s0+20o Mesh. l3. 17 4s. 5 10. 2 -2n0+s25 Mesh. 25.18 57. 7 25. s 4425 Mesh 47. 91 66. 7 56. 6

In order for this typical concentrate to contain fthe desired maximum impurity content of. 10'% S102, it is necessary to raise the iron content from the value of 56.3% shown in Table I to approximately 64.8% Fe; as has been previously 3 related, this is commonly accomplished by further grinding and additional magnetic separation. For the type example shown, I the additional grinding requiredwonld be to a roximately 150 mesh with 80% passing a 325 mes screen.

In order to obviate the expense of additional grinding, we propose to eliminate the low grade middling particles by utilizing the settling rate difi'erential between high grade magnetite and low grade middling particles. Such a separation between high specific gravity particles and low specific gravity particles of approximately the same diameter can readily be accomplished by hydraulic classification, wherein the liquid entering the classifier is adjusted to the pointwhereat the velocity of the rising current is large enough to cause the lighter weight particles to overfiow, but not so large as to cause the heavier weight particles also to overflow. In other words, particles of lesser specific gravity rise with the current of liquid in the classifier while particles of greater density sink in the current and are removed from the classifier at a separate point.

When this method is applied to a suitable product such as a 65 mesh magnetic concentrate, diillculty is encountered because of the range in size of the particles comprising the concentrate; thus high grade magnetite particles of small size are transported upward by the current required for overflowing coarser sizes of low grade middling particles; this occurs even if the material be premagnetized to form the magnetic fines into clusters for it is impossible to cluster the fines to an extent whereat these are not carried out by the current which removes the coarse middling grains. For this reason it has been impossible heretofore tomake a clean separation between the high grade fines and low grade middlings.

Another presently employed flowscheme includes grinding the ore in a ball mill-classifier circuit, which includes magnetic concentrators treating either the clasifier sands or the ball mill discharge. The theory underlying this practice is that barren gangue particles are eliminated from the circuit as they are liberated. The classifier overflow goes to additional magnetic concentrators for further treatment because the silica content is still too high to satisfy the requirements. This practice may possess certain advantages over that first described, but it also requires excessive grinding of the valuable iron minerals. Results on a typical low grade magnetic iron ore treated in accordance with this process show that the classifier overflow had to be extremely fine if a desired grade of about 65% iron with about SiOz was to be obtained. With both these prior methods the presence of extreme fines is very undesirable as it increases the dimculties and the costs of the agglomerating process which must be subsequently practiced on the concentrate. With our invention it is possible to produce a concentrate of equally high grade, but containing a smaller amount, of extreme fines. This is shown in the following example; in both cases the same ore is being treated;

4 Thus the concentrate typical of those operations heretofore available included 95.6% of material passing a 200 mesh screen, whereas in the concentrate produced in accordance with this invention from the same ore only passed a 200 mesh screen; also, only 19.7% of the old practice concentrate was coarser than 325 mesh while 45% of our concentrate from the same ore was coarser. Obviously, the coarser concentrate can be agglomerated at lower cost, with greater facility and less dust loss.

In accordance with the process of our invention, a sharper and cleaner'separation is made possible by the following technique:

The concentrate to be processed in accordance with our invention can be prepared by any suitable process and in any acceptable equipment and, as will presently appear, we have disclosed two difierent procedures for providing such a concentrate. The particles comprising the concentrate should be of such size that the economic maximum of iron oxide is present as free particles. The suitably prepared magnetic concentrate is first demagnetized and is then tractionated into several products, each of which is graded according to particle size and specific gravity. Each separate fraction is then re-magnetized as by passing it through a strong direct current field, so that the high grade particles form clusters. Each magnetized fraction is again fractionated hydraulically, the low grade middling particles being caused to overflow and become separated from the high grade material remaining in the hydraulic classifier. Magnetization of the classified products before removal of the low grade middling particles causes the relatively fine grains of high grade magnetite to cluster together; in efiect, these clusters act approximately as single particles of larger size, thereby reducing the tendency for the high grade magnetite to overflow with low grade middling. Hydraulic sizing prior to magnetic clustering also aids in establishing the correct conditions for minimum loss or iron values. We have found that by grinding the ore to. a relatively coarse size of the order of a 65 mesh screen size, a suitable concentrate of acceptable iron and silica content can be achieved without further grinding; heretofore it has been deemed essential to comminute the first concentrate made from the first comminution (all passing a 65 mesh screen) to a very finely divided form, of the order whereat all particles pass a mesh screen. This invention enables the second comminution to be omitted.

It is an object of this invention to provide a process for bencficiation of an iron ore into' a concentrate of acceptable iron and silica con- 'tent.

Another object of this invention is to provide a novel process for beneficiation of an iron ore.

In the drawings, Figures 1 and 2 are diagrammatic flow sheets illustrating processes embodying our invention.

As a typical operating example illustrative of the practice of the present invention, a suitable iron ore was processed in a system which, as is shown in Figure 1 in the accompanying drawing, included a rod mill, a suitably pre-crushed ore being delivered to this mill. The output from the mill was sent to a magnetic cobber, the non-magnetic fraction being discharged to waste and the magnetic fraction sent on to a classifier ball mill circuit wherein all of the material was reduced to size whereat it would pass a screen of -a,uss,oso

.5 predetermined mesh. From vthe classifier, th ground material was sent on to a magnetic separator and a further separation made, the magnetized product being separated and then demagnetized. The demagnetized product was then delivered to a hydraulic classifier wherein a plurality of different size products were derived. Each fraction was then separately magnetized and subjected to separate hydraulic classification, the concentrate being taken off and the tailings separated and taken off for discard. Our test-work has shown the concentrate produced by this system to be of acceptable iron and silica content.

As a rod mill, cobber, mill-classifier, magnetic separator, etcetera, one can employ equipment well known to those skilled in the art of ore beneficiation.

In a typical operation, 3,200 tons perday of ore. crushed to a half-inch and containing 35.1% total iron, 30.6% magnetic iron and 21% silica, would be delivered to a rod mill wherein the material is reduced to approximately a 14 mesh size. This material is then delivered to a magnetic cobber wherein 607 tons perday are taken off as tailings and a concen trate, amounting to 2,693- tons per day, is delivered to a classifier-ball mill system. The product from this system, all of a size passing a 65 mesh screen, is delivered to a magnetic separator wherein the concentrate is further reduced to 1,835 tons per day, 858 tons per day being removed as tailings. The concentrate is then subject 'to demagnetization; the product at this point contains 56.7 total iron and is all of a size passing 65 mesh. After separation into five separate fractions, four spigot fractions and an overflow fraction, the fractions amounting respectively to 172, 423, 169, 426 and 655 tons per day; each fraction is separately magnetized and is then sent into a hydraulic classifier from which 1,537 tons per day of a combined concentrate are taken. The results are shown in further detail in the Table 11:

TABLE II fractions of mesh magnetic concentrate Wt. Per Wt. Ier Distribu- Cent Cent 6 tion Fe Product Heads um 5 Per Cent Overflow 9. 30 42. 0 3. 905 6. 88 59. 4 5. 52 60. 0 3, 312 5. 84 40.6 3. 78 13. 7 0.593 1.04 22. 61. 2 14. 734 25. 97 02. 5 21. 18 68. 1 14. 424 25. 42 7.7 1.7 17.3 0.310 0.55 9. 17 65. 2 5. 981 10. 54 93. 5 8. 57 68.1 5. 836 10. 28 6. 5 0. 60 24. 1 0. 0. 26 23. 09 59. 8 13. S16 24. 34 92. 2 21. 29 63. 9 13, 604 23. 97 7.8 1.80 11. 8 0.212 0.37 35. 49 51. 6 18. 315 32. 27 75. 4 26. 76 62. 2 10. 645 29. 32 5. 0 l. 77 27. l 0. 480 0. 85 19.6 6.96 17.1 1.190 2.10 100. 00 56. 8 56. 751 100. 00

Compo. Cone. I 83. 32 64. 6 53. 821 94. 83 Mids and Tail. 16. 68 17. 6 2. 930 5.17

Summarizing the benefits gained by this proabove table.

1 "Per cent Iron 'content'of concentrate before treat-' ment by hydraulic'fractionation and mag- The same technique can be applied to an ore concentrate prepared by other methods,,for example, in accordance with the flow sheet shown in Figure 2 wherein the crushed ore is passed to a rod mill, the product being magnetically segregated and the demagnetized product is sent to a classifier from which the sand return is sent to a ball mill-magnetic separator circuit, the prod not being returned to the demagnetizer-classifier line. The classifier overflow is hydraulically sized in a second classifier into a pluralit of sizes, each size being magnetized and classified again hydraulically under proper Ve10cities of classifying upward current for each size. When this technique is used, a much coarser grind can be employed and the desired grade of finished product can be obtained without the use of additional grinding or magnetic concentrators.

In addition to eliminating the unnecessary waste of power, our invention brings out another advanta e. In both the previous fiowschemes described as in use prior to this invention, the final product from the plant was extremely fine. It is necessary that this product be agglomerated. either by sintering on a machine such as a Dwight & Lloyd Sinterer or modulized in a rotary kiln.

Using the fiowscheme employed prior to this invention, the classifier overflow was practically all minus mesh and 83% minus 325 mesh, the final magnetic concentrate from the plant analyzed 64.48% Fe and 9.90% SiOz. When the grind was coarsened so that the classifier overflow con-, tained 65% of minus 325 mesh material, in an attempt to reduce dust loss and other undesirable factors, the final product fell to 62.3% Fe and 13.02% S102. As we have brought. out heretofore, one can produce by our invention an acceptable concentrate from the same ore which contains only 55% .of minus 325 mesh so that agglomeration is readily and economically practiced.

From the foregoing it should be obivous that one can replace an expensive comminution operation with the relatively inexpensive demagnetization, hydraulic classification re-magnetization and final hydraulic concentration to produce a concentrate of acceptable iron-silica content.

We claim:

1. A process for treating a magnetic iron ore comprising grinding the ore to finely divided form, separating non-magnetizable particles from the finely divided ore, demagnetizing remaining finely divided ore, hydraulically classifying the demagnetized ore into at least three different size fractions, magnetizing each of said fractions, and hydraulically classifying each of said fractions into a tailing fraction and an iron rich fraction.

2. A process for treating a magnetic iron ore comprising grinding the ore to a coarsely divided form, removing a non-magnetic fraction from the coarse ore, grinding the remaining ore to finely divided form, separating non-magnetizable particles from the finely divided ore, demagnetizing remaining finely divided ore, hydraulically classifying the demagnetized ore into at 7 least three of different size fractions, magnetizing each of said fractions, and hydraulically classifying each of said fractions into a tailing fraction and an iron rich fraction.

3. A process for treating a magnetic iron-silica ore in a finely divided form comprising hydraulically classifying said ore in a demagnetized con dition into at least three separate fractions, passing each fraction through a magnetic field to assemble fine particles rich iniron into clusters, and hydraulically classifying each magnetized fraction into a tailing fraction and an iron rich fraction.

4. A process for treating an iron-silica ore comprising grinding the ore, separating the ore magnetically into a magnetized and a non-magnetized fraction, demagnetizing the magnetized fraction, classifying the demagnetized fraction DMITRI N. VEDENSKY. LESLIE J. BECHAUD, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STA FEES PATENTS Number Name Date 1,286,247 Davis Dec. 3, 1918 2,276,075 Wuensch M3.1'. 10, 1942 2,388,471 De Vaney -Q Nov. 6, 1945 2,468,586 Braund Apr. 26, 1949 OTHER REFERENCES Bulletin of the University of Minnesota, Magnetic Concentration of Iron Ore, by E. W. Davis, vol. XXIV, Book #606, No. 43, December 21, 1921, page 123. 

1. A PROCESS FOR TREATING A MAGNETIC IRON ORE COMPRISING GRINDING THE ORE TO FINELY DIVIDED FORM, SEPARATING NON-MAGNETIZABLE PARTICLES FROM THE FINELY DIVIDED ORE, DEMAGNETIZING REMAINING FINELY DIVIDED ORE, HYDRAULICALLY CLASSIFYING THE DEMAGNETIZED ORE INTO AT LEAST THREE 